The circuit breaker is generally composed of contact system, arc extinguishing system, operating mechanism, tripping device, etc. The role of the circuit breaker is to cut off and connect the load circuit, to cut off the fault circuit, to prevent the expansion of the accident and to ensure safe operation. The circuit breaker has the following features: rated voltage, rated current In, setting range of tripping current for overload protection and short circuit protection, rated short-circuit breaking current (industrial circuit breaker or Household circuit breaker), etc.. When they occur serious overload or short circuit and under-voltage fault can automatically cut off the circuit, which function is equivalent to the switch and the combination of over/under heat relay. Circuit breakers have been widely used.